Non-destructive inspection (NDI)/non-destructive evaluation (NDE) (hereinafter referred to individually and/or collectively as NDE) of structures involves thoroughly examining a structure without harming the structure or requiring significant disassembly of the structure. Non-destructive inspection may be advantageous to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or inconsistencies in the structure. Inspection may be performed during manufacturing of a structure and/or once a structure is in-service. For example, inspection may be required to validate the integrity and fitness of a structure for continued use in manufacturing and future ongoing use in-service. However, access to interior surfaces is often more difficult or impossible without disassembly, such as removing a part for inspection from an aircraft.
Among the structures that may be non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies. In this regard, composite structures are used throughout the aircraft industry because of the engineering qualities, design flexibility and low weight of composite structures, such as the stiffness-to-weight ratio of a composite sandwich structure. As such, it may be desirable to inspect composite structures to identify any anomalies, such as cracks, voids or porosity, which could adversely affect the performance of the composite structure. For example, anomalies in composite sandwich structures, generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to the opposed sides of the core, may include disbonds which occur at the interfaces between the core and the skin or between the core and a septum intermediate skin.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar anomalies and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors may be used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft and other structures may be performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. While solid laminates may be inspected using one-sided pulse echo ultrasonic (PEU) testing, composite sandwich structures typically require through-transmission ultrasonic (TTU) testing for high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and a receiver sensor, are positioned facing the other but contacting opposite sides of the structure to be inspected such as opposite surfaces of a composite material. An ultrasonic signal is transmitted by at least one of the transducers, propagated through the structure, and received by the other transducer. Data acquired by sensors, such as TTU transducers, is typically processed by a processing element, and the processed data may be presented to a user via a display.
In order to couple the ultrasonic signals into the structure under inspection, a couplant may be utilized between the transducer and the surface of the structure. In TTU systems having both a transmission-side transducer and a receiver-side transducer, a couplant may be disposed between each of the transducers and the respective surfaces of the structure. In order to couple sufficient energy into the structure to permit the structure to be inspected with a desired sensitivity, TTU systems may utilize water as the couplant. While the water may effectively couple the ultrasonic signals into the structure under inspection, a water delivery and removal system must be provided in order to deliver the water to the space between the transducer and the surface of the structure and to collect the excess or unused water. Not only do such water delivery and removal systems add to the expense of an NDE system, but a water delivery and removal system may make the positioning and movement of a water-coupled NDE system more cumbersome.
Further, it may be undesirable for some structures to be placed into contact with water, thereby limiting the usefulness of a water-coupled NDE system for the evaluation of such workpieces. In this regard, it may be desirable to inspect workpieces during manufacture such that the workpiece is in an incomplete form, such as a partially cured laminate or a honeycomb or foam core prior to the application of a skin thereto. Although the NDE of such incomplete structures may be desirable, it may not be advisable to expose such incomplete structures to water since the water may have an adverse impact upon the partially cured laminate or the honeycomb or foam core. Thus, water-coupled NDE techniques are not generally practical in conjunction with the inspection of such incomplete structures.
As noted above, the water delivery and removal system can make the positioning and movement of a water-coupled NDE system more cumbersome. Moreover, in some instances, at least one of the transmission-side transducer or the receiver-side transducer must be relatively small, such as in order to be inserted through a relatively small opening and/or to be moved along an interior surface of a structure in which little room is provided for movement of the TTU unit. For example, there is interest in surgical NDE systems in which at least one of the TTU units is inserted through a relatively small opening and is then moved through a relatively small space with limited accessibility. In these applications, a water couplant may be unworkable since it may be difficult, if not impossible, to both appropriately deliver and remove the water in instances in which the transducer is internal to the workpiece.
Air-coupled TTU inspection systems have also been developed in which the transmitter-side transducer and the receiver-side transducer are coupled via a layer of air to the workpiece. For ultrasonic signals, however, air does not couple the signals as efficiently as does water due to the substantial mismatch in acoustic impedance between the air and the materials of the transducer and the workpiece which leads to high interfacial reflection and low acoustic transmission efficiency. As such, in order to air-couple ultrasonic signals having sufficient energy into a workpiece such that resulting signals could be reliably detected by the receiver-side transducer, the ultrasonic signals were of a relatively low frequency, such as about 50 kHz, and therefore had only limited sensitivity. As a result of the limited sensitivity, the use of such air-coupled NDE systems is of marginal, or no, use for the inspection of workpieces that require greater sensitivity, such as during manufacturing operations and/or during in-service inspections.
It would therefore be desirable to provide improved NDE techniques including NDE techniques that rely upon the through transmission of ultrasonic signals. In particular, it would be desirable to provide for improved TTU inspection techniques that facilitate the inspection of workpieces that have at least one surface of limited accessibility, thereby facilitating the surgical NDE of a workpiece. It would also be desirable to provide for improved TTU inspection without exposing the workpieces to water, which may be deleterious to the workpiece or at least complicate further processing of the workpiece.